Because there is no heating spiral, these lamps are particularly suited for a long service life in a harsh environment. These lamps are used preferably in automobile design, for example, due to their lack of sensitivity to impacts and shaking.
The design of this illuminant allows it to be used for irregular geometries; thus tube-shaped designs with an outer diameter of 3-6 mm and a length of a few decimeters, for example, are known. The designs can be manufactured to be curved, coiled or meandering.
To start the discharge process, a high voltage, which can amount to several thousand Volt, is needed. The burn voltage after firing can still amount to several hundred Volt.
The necessary burn current is in a range between 1 to 20 mA, depending on the design of the lamp geometry.
The lamp is started simply by applying high-tension a.c. voltage, which may--as soon as the firing procedure is completed--be broken down to the burn voltage via the initial current.
The characteristic curve of the luminous intensity in relation to the temperature necessitates that the lamp burn current be tracked under certain extreme environmental conditions.
Since this characteristic curve does not run linearly and even exhibits turning points, it is only possible to fulfill the requirement for a constant light progression in the specified temperature range with great expense.
Due to the increasing use of cold cathode discharge lamps in the illumination of instruments in automobile design, a large setting range of brightness is required in order to ensure a corresponding darkening or dimming of the lighting during changing environmental brightness. Darkening values and dim rates from 1 to 1000 are required.
The firing voltages and burn currents are generally made available by means of varying transformatoric means. Solutions with freely-oscillating flyback converters or semiconductor-switched push-pull converters are known, which are self-controlled according to the current saturation principle and adjust to the respective operating conditions. One example of such circuitry is known from U.S. Pat. No. 5,053,681, whereby relatively high-frequency operating currents from the a.c. voltage supply in the system are generated and distributed to feed the fluorescent lamps. However, these converters are very costly and inefficient for use in the broad voltage range needed in automobile design. Additionally, the control option for this type of converter is severely limited. Darkening and dimming are only possible by altering the voltage. Examples of this design are known from DE-A-42 04 020 and WO-A-83 02 537. The first specification relates to the phase control of neon tubes in luminary advertisements; the second specification comprises a fluorescent lamp ballast for gas-discharge lamps in which the brightness control of the lamps is achieved by altering the operating frequency.
If, however, to illuminate an instrument panel, for example, a plurality of lights with varying geometries and thus variable firing and burn voltages is used, and if the individual lamps are mounted far apart from one another, then an individual voltage source would be needed for each lamp. A joint dimming with the goal to darken or brighten all lights synchronously in the same way and with equal intensity can only be achieved via great expense and losses or is not possible at all.
A large number of freely-oscillating converters would make it very difficult or impossible to sufficiently eliminate interference in a high-frequency range with the varying frequencies for the various operating conditions.
Major problems are also caused by arranging a plurality of high-voltage lines parallel from one or a plurality of converters to the remotely-located lamps, particularly if the a.c. voltages are selected in the kHz range and these voltages have a phase relationship that deviates from one another.